I was trying to test some other aspect of the game. So I made a cobblestone fence around a zombie far far far away from where I originally spawned (cuz I appeared in a jungle). Then I set my new spawn point next to the fence. I donned some diamond armor and a sword and let the zombie kill me in survival mode. When I respawned next to the fence the zombie was gone as well as most of my items. I then proceeded to jump over the cobblestone fence (which probably shouldn't happen either) afterwards of which, I picked up all of my invisible items, then got attacked and killed by an invisible zombie.
I saved and exited into the main menu and returned to the map. The zombie was visible again, but when it killed me, it disappeared again. I managed to recreate this and record it. The vid can be found here---> http://www.youtube.com/watch?v=yskAreLIF0g&feature=plcp

to recreate:
1) start a new game in creative.
2) fly WAY FAR AWAY from the spawn point.
3) create a new spawn point at your location with "/spawnpoint"
4) use "/gamemode 0"
5) get killed by a mob
6) get attacked by invisible monsters